TITLE: GCN CIRCULAR NUMBER: 14168 SUBJECT: GRB 130131A: Swift-XRT refined Analysis DATE: 13/02/01 02:07:03 GMT FROM: Phil Evans at U of Leicester J.A. Kennea (PSU), M.C. Stroh (PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), O.M. Littlejohns (U. Leicester), A. Maselli (INAF-IASFPA), V. Mangano (INAF-IASFPA), A. Melandri (INAF-OAB) and D. Grupe report on behalf of the Swift-XRT team: We have analysed 9.2 ks of XRT data for GRB 130131A (Grupe et al. GCN Circ. 14156), from 64 s to 18.8 ks after the BAT trigger. The data comprise 235 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN. Circ 14160). The late-time light curve (from T0+5.3 ks) can be modelled with a power-law decay with a decay index of alpha=1.1 (+0.4, -0.3). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.44 (+/-0.07). The best-fitting absorption column is 3.17 (+/-0.19) x 10^21 cm^-2, in excess of the Galactic value of 1.4 x 10^20 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index of 2.21 (+0.21, -0.20) and a best-fitting absorption column of 2.4 (+/-0.6) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (6.1 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.4 (+/-0.6) x 10^21 cm^-2 Galactic foreground: 1.4 x 10^20 cm^-2 Excess significance: 6.5 sigma Photon index: 2.21 (+0.21, -0.20) If the light curve continues to decay with a power-law decay index of 1.1, the count rate at T+24 hours will be 2.6 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.3 x 10^-14 (1.6 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00547407. This circular is an official product of the Swift-XRT team.